Active implantable medical devices include circuits for detecting cardiac activity, i.e., the detection of spontaneous depolarizations of the myocardium, as well as circuits for stimulating the myocardium, e.g., in the absence of a spontaneous depolarization. After a stimulation, it is important to be able to collect (sense) the “evoked wave”, i.e., the depolarization wave that is induced by the stimulation of the considered cavity, in order to determine whether the stimulation was effective and, for example, to adjust the amplitude and/or the width of the stimulation pulse as appropriate to cause an effective depolarization.
One difficulty in this arises from the fact that the evoked wave is very early, i.e., it occurs very soon after the stimulation pulse is delivered and that at the detection amplifier output it is mixed in, and can be obscured with, an electric transient called the “amplifier response” consecutive to the electric stimulation. This amplifier response is always present independent of the presence or absence of an evoked wave, and depends only on the electric characteristics of the amplifier itself, of the energy contained in the stimulation pulse (amplitude and width), as well as of the impedance characteristics of the heart/electrode interface, called “load polarization”.
In particular, to eliminate the electrical loads at the heart/electrode interface after a stimulation, one envisages to carry out a discharge of any accumulated energy. If this is not done, the following stimulations would end up not being effective. During the phase of discharge, it is envisaged to disconnect the amplifier inputs from the electrode terminal, which is known as a “blanking” of the detection circuits, typically for a length of time of about 14 ms, the stimulation pulse having a duration of about 1 ms. Moreover, at the time of the re-connection of the amplifier inputs to the terminals of the detection electrode at the end of the blanking period, a transitory rebound voltage appears at the amplifier output, which last a few milliseconds until the amplifier is completely de-saturated.
The study of the evoked wave is even more difficult in the case of an atrial stimulation. Indeed, an evoked atrial (P wave) wave has an amplitude that is much lower than in the case of the ventricle stimulation (R wave) and moreover occurs much sooner after stimulation: thus, the atrial evoked wave (P wave) appears approximately 10 ms after a stimulation to finish at approximately 30 ms, whereas the ventricular evoked wave (R wave) is observed approximately 60 ms after a ventricular stimulation.
One will understand that, under these conditions, it is very difficult to detect the presence of an evoked P wave, for example, in the case of an atrial capture test.